1. Field of the Invention
The present invention relates to a switching power supply unit having a main-switching element and one or more sub-switching elements, which perform ON/OFF operations in synchronism with the ON/OFF operations of the main-switching element or opposite to.
2. Description of the Related Art
In general, in electronic equipment such as electronic calculators and communication devices, a switching power supply unit has been widely used for supplying a stabilized DC voltage from a commercial AC power supply. Such a switching power supply unit may comprise in each circuit system such a forward converter or a flyback converter, in which a main-switching element, which is connected in series to a primary winding of a transformer, is repeatedly turned on/off, and then an input voltage is intermittently applied to the transformer so as to obtain a DC output through a rectifying/smoothing circuit connected to a secondary winding. In contrast with such switching power supply units, there are provided switching power supply units, in which improvement in circuit characteristics can be achieved by adding a circuit which includes a sub-switching element performing ON/OFF operations in synchronism with or opposite to the ON/OFF operations of a main-switching element.
A description will be given of a structure of a conventional switching power supply unit having such a sub-switching element, referring to the drawings.
First, a conventional unit disclosed in Japanese Unexamined Patent Publication No. 8-317647 will be illustrated, referring to FIG. 26.
In this figure, numeral 50 represents a switching power supply unit, which has a partial-resonance converter circuit 51 and a drive circuit 52. Of these components, the partial-resonance converter circuit 51 comprises capacitors C51, C52, C53 and C54, diodes D51, D52 and D53, a transformer T51, a main-switching element S51 and a sub-switching element S52.
In addition, the drive circuit 52 comprises an output control circuit 53, comparators 54 and 55, an inverter 56, an isolating circuit 57, a triangle-wave oscillator 58, a light-emitting-side photocoupler Pa, a light-receiving-side photocoupler Pb, a transistor Q51, and resistors R51, R52, and R53.
In the switching power supply unit having the above-described structure, the sub-switching element S52 performs ON/OFF operations in reverse to the ON/OFF operations of the main-switching element S51.
Next, another conventional circuit disclosed in Japanese Unexamined Patent Application No. 8-37777 will be illustrated with reference to FIG. 27.
In this figure, numeral 60 represents a switching power supply unit, in which an FET disposed on the secondary side of a transformer is used for rectification. This is referred to as the so-called synchronous rectification system. The switching power supply unit 60 comprises a transformer T61, an input capacitor C61, an FET Q61 as a main-switching element, an FET Q62 as a sub-switching element, similarly, an FET Q63 as another sub-switching element, a choke coil L61, an output capacitor C62, a light-emitting-side photocoupler PA, a light-receiving-side photocoupler PB, comparators 61, 62 and 63, a triangle-wave oscillator 64, isolating circuits 65 and 66, an inverter 67, a control circuit 68, and a control-signal output circuit 69. Of these constituent parts, the control-signal output circuit 69 comprises transistors Q64 and Q65, and resistors R61 to R65.
In the switching power supply unit 60 having the above-described structure, the FET Q62 performs ON/OFF operations in synchronism with the ON/OFF operations of the FET Q61, whereas the FET Q63 performs ON/OFF operations in reverse to the ON/OFF operations of the FET Q61.
In each of the above-described switching power supply units, however, the circuit driving the sub-switching element is formed of an IC. Additionally, since a ground level is different between the main-switching element and the sub-switching element, the installation of an IC and an isolating circuit comprising a photocoupler is necessary. As a result, the use of an IC, a pulse transformer, or the like, leads to complications in the circuit structure and increase in production cost. Furthermore, such an increase in the number of parts can be a hindrance to miniaturization and weight reduction.